Tunable voltage controlled oscillator

ABSTRACT

A voltage controlled oscillator incorporating a ferroelectric capacitor in its resonant circuit is provided in order to provide superior phase noise performance and a linear control voltage/capacitance relationship. The resonant circuit may include multiple ferroelectric capacitors and multiple control voltages in order to provide band switching capability and/or increase the tuning range of the oscillator. The feedback loop of the oscillator may also incorporate a ferroelectric capacitor in order to adaptively optimize the feedback.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/283,093, filed Apr. 11, 2001, which is herebyincorporated by reference. In addition, this application relates to thefollowing U.S. applications, which are hereby incorporated by reference:Ser. No. 09/904,631 filed on Jul. 13, 2001, by Stanley S. Toncichentitled “Ferro-Electric Tunable Filter”; Ser. No. 09/912,753 filed onJul. 24, 2001 by Stanley S. Toncich entitled “Tunable Ferro-ElectricMultiplexer”; Ser. No. 09/927,732 filed on Aug. 8, 2001, by Stanley S.Toncich entitled “Low Loss Tunable Ferro-Electric Device and Method ofCharacterization”; Ser. No. 09/927,136 filed on Aug. 10, 2001, byStanley S. Toncich entitled “Tunable Matching Circuit”; Ser. No.10/044,522 filed on Jan. 11, 2002, by Stanley S. Toncich entitled“Tunable Planar Capacitor”; Ser. No. 10/077,654 filed on Feb. 14, 2002,by Stanley S. Toncich entitled “Tunable Isolator Matching Circuit”; Ser.No. 10/076,171 filed on Feb. 12, 2002, by Stanley S. Toncich entitled“Antenna Interface Unit”; Ser. No. 10/075,896 filed Feb. 12, 2002, byStanley S. Toncich entitled “Tunable Antenna Matching Circuit”; Ser. No.10/075,727 filed Feb. 12, 2002, by Stanley S. Toncich and Tim Forresterentitled “Tunable Low Noise Amplifier”; Ser. No. 10/075,507 filed onFeb. 12, 2002, by Stanley S. Toncich entitled “Tunable Power AmplifierMatching Circuit”.

FIELD OF THE INVENTION

[0002] The present invention relates generally to tunable electronicdevices and components and, more particularly, to voltage controlledoscillators incorporating tunable ferroelectric components.

BACKGROUND OF THE INVENTION

[0003] Radio frequency bandwidth is a scarce resource that is highlyvalued and is becoming increasingly congested. Ever-increasing numbersof users are attempting to co-exist and to pass ever-increasing amountsof information through the finite amount of bandwidth that is available.The radio spectrum is divided into frequency bands that are allocatedfor specific uses. In the United States, for example, all FM radiostations transmit in the 88-108 MHz band and all AM radio stationstransmit in the 535 kHz-1.7 MHz band. The frequency band around 900 MHzis reserved for wireless phone transmissions. A frequency band centeredaround 2.45 GHz has been set aside for the new Bluetooth technology.Hundreds of other wireless technologies have their own band of the radiospectrum set aside, from baby monitors to deep space communications.

[0004] Communications within a given frequency band occur on even morenarrowly and precisely defined channels within that band. Hence, invirtually any wireless communication system or device, frequency agilityis required and accurate frequency generation is of critical importance.Frequency generation is typically provided by an electronic oscillator.As is well known in the art, an electronic oscillator is a circuit thatproduces an output signal of a specific frequency, and consistsgenerally of an amplifier having part of its output returned to theinput by means of a feedback loop. A very simple electronic oscillatorincludes some combination of a capacitor with an inductor or otherresonator.

[0005] The capacity for frequency channel selection and changing can beprovided by a voltage controlled oscillator (“VCO”). In a VCO, a controlvoltage is applied to a voltage dependent capacitor, commonly referredto as a variable capacitance diode, varicap diode or varactor, in orderto tune the VCO to a particular frequency. FIG. 1 illustrates aconventional varicap diode tuned oscillator resonant circuit 100.Circuit 100 includes varicap diode D1 and resonator L1 (L1 is aninductor or some other form of resonant transmission line device).Control voltage V1 is applied across varicap diode D1 via inputresistance R1. V1 is a DC control voltage and is applied to tune theoscillator over a specified range. DC blocking capacitor C1 isinterposed between varicap diode D1 and inductor L1, and DC blockingcapacitor C2 is interposed between inductor L1 and an oscillatorsustaining amplifier (not shown). Typically, the sustaining amplifier isa negative impedance generator. As is well known in the art,phase-locked loop (PLL) control circuitry will also typically beprovided in conjunction with the VCO.

[0006] When a reverse voltage (V1) is applied to varicap diode D1, theinsulation layer between the p-doped and n-doped regions of thesemiconductor thickens. A depletion region that is essentially devoid ofcarriers forms in diode D1, and behaves as the dielectric of thecapacitor. The depletion region increases as the reverse voltage acrossit increases, and since capacitance varies inversely as dielectricthickness, the junction capacitance decreases as the reverse voltageincreases. The effect is similar to separating the two plates of acapacitor by a larger distance, which decreases the capacitance. So, byvarying the control voltage VI the junction capacitance provided byvaricap diode D1 can be varied. Varying the capacitance, in turn,changes the resonant frequency of inductor L1 and hence the frequencythat will be amplified and output by circuit 100.

[0007] In recent years, VCO designers have been required to comply withsignificantly more demanding specifications. Currently, only a handfulof manufacturers world wide can economically produce VCOs that aresuitable for use in high volume consumer communication devices. Two ofthe major hurdles faced in VCO design are (1) phase noise and (2) theinherent non-linear transfer function (applied voltage versuscapacitance) of varicap diodes.

[0008] One critical parameter of oscillator performance is its singlesideband phase noise, or simply “phase noise”. Phase noise affects thereceiver's ability to reject unwanted signals on nearby channels. It isthe ratio of the output power divided by the noise power at a specifiedoffset and is expressed in dBc/Hz. FIG. 2 is a graph showing the typicalphase noise requirement for a 1 GHz VCO. As can be seen, at an offset ofabout 60 kHz a 1 GHz oscillator specifies a phase noise of about −120dBc/Hz.

[0009] One of the main stumbling blocks to achieving this performance isthe loaded Q of the oscillator circuit. The sustaining amplifier of theoscillator does not usually play a significant factor in phase noisedetermination due to the availability of low noise semiconductors thatare specifically optimized for this purpose. The loaded Q of theresonator structure (L1) is typically the dominant factor in determiningthe overall phase noise performance. The loaded Q of the resonator isfrequently limited by the series resistance of the varicap diode, whichcan be as much as several ohms.

[0010] The Q of a capacitor can be expressed by:

[0011] Q=X_(c)/R_(s), where X_(c) is the reactance of the varicap diodegiven by X_(c)=1/(2·π·f·c),

[0012] and R_(s) is the effective series resistance of the varicapdiode.

[0013] If a required capacitance of 5 pF at a frequency of 1.5 GHz isassumed, a reactance X_(c) of 21.22 Ω results. If it is further assumedthat the effective series resistance R_(s) of the varicap diode is 0.5Ω, the resultant Q of the varicap diode is 42.44. Hence, reducing theeffective series resistance will have a direct impact on the Q of thevaricap diode and the loaded Q of the entire resonator structure.

[0014] Another critical parameter in oscillator performance is thelinearity (or lack thereof) in the transfer function (applied voltageversus capacitance) of the varicap diode. FIG. 3 is a chart plotting thecapacitance of a typical varicap diode versus a typical tuning voltagerange for the diode in a mobile phone (0.3V to 2.7V). As can be seen, itis not a linear relation. Below 0.5V, unit voltage changes lead to muchgreater unit capacitance changes. Consequently, the MHz/volt frequencyshift of the oscillator is not constant across the tuning range. Thisleads to compromise in the design of the PLL loop filter and thusoverall noise performance.

[0015] Another problem associated with the use of varicap diodes isthat, since it is a reverse-biased diode junction, it is important thatthe applied AC signal does not overcome the bias voltage and result inheavy forward conduction of the diode. If this occurs the Q of theresonator will be dramatically lowered and various oscillator parameterssuch as phase noise and general spectral purity will be seriouslyimpacted. In an extreme case, the oscillator may fail to maintain acontinuous oscillation and degenerate into parasitic uncontrolled burstoscillations.

[0016] In view of the above, there is a need for a voltage controlledoscillator that exhibits better phase noise performance and a morelinear voltage/capacitance transfer function.

SUMMARY OF THE INVENTION

[0017] The present invention provides a voltage controlled oscillatorthat incorporates a tunable ferroelectric capacitor to provide betterphase noise performance and a more linear voltage/capacitance transferfunction.

[0018] Accordingly, in one embodiment of the invention, a voltagecontrolled oscillator is provided that has a resonant circuit forgenerating a tuning frequency. The resonant circuit comprises aninductive element and a ferroelectric capacitor having a variablecapacitance. A control line is coupled to the ferroelectric capacitorfor applying a control voltage to the capacitor to vary the capacitancewhich, in turn, varies the tuning frequency of the resonant circuit.

[0019] In another embodiment of the invention, a voltage controlledoscillator is provided. The oscillator includes a resonant circuithaving a first variable ferroelectric capacitor to generate a signalhaving a variable resonant frequency, and an amplifier coupled to theresonant circuit to amplify the signal. A feedback loop coupled betweenthe amplifier and the resonant circuit incorporates a secondferroelectric capacitor to control the amplitude and phase of a feedbacksignal.

[0020] Another embodiment of the invention comprises a band-switchableoscillator resonant circuit. The circuit has first and secondferroelectric capacitors and first and second control voltage lines tofacilitate band switching.

[0021] The present invention also provides a method for band switchingin a voltage controlled oscillator. First and second ferroelectriccapacitors are provided, and first and second control voltages areapplied to the first and second capacitors so that either the firstcapacitor or the second capacitor dominates the output frequency of theoscillator.

[0022] Other features, objects and implementations of the invention willbe or will become apparent to one with skill in the art upon examinationof the following figures and detailed description. All such additionalfeatures, objects and implementations are intended to be included withinthis description, to be within the scope of the invention and to beprotected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic of a conventional varicap diode tunedoscillator resonant circuit.

[0024]FIG. 2 is a graph plotting phase noise versus frequency offsetrequirements for a typical 1 GHz voltage controlled oscillator.

[0025]FIG. 3 is a graph plotting capacitance versus applied controlvoltage for a typical varicap diode.

[0026]FIG. 4 is a schematic of a ferroelectric tuned oscillator resonantcircuit according to the present invention.

[0027]FIG. 5 is a graph plotting capacitance versus applied controlvoltage for a ferroelectric capacitor according to the presentinvention.

[0028]FIG. 6 is a schematic of a ferroelectric tuned oscillator circuitwith adaptive feedback according to the present invention.

[0029]FIG. 7 is a schematic of a band-switchable ferroelectric tunedoscillator resonant circuit according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] VCOs currently in use typically employ a varicap diode as afrequency tuning device. These VCOs exhibit compromised phase noiseperformance and a non-linear capacitance/control voltage transferfunction. In order to overcome these shortcomings, the present inventionprovides a voltage controlled oscillator employing a ferroelectriccapacitor, rather than a varicap diode, as the tuning device.

[0031] The background, advantages, topologies and test methodsassociated with ferroelectric capacitors are fully set forth in commonlyowned U.S. provisional application serial No. 60/283,093 filed on Apr.11, 2001, as well as commonly owned related application Ser. Nos.09/904,631; 09/912,753; 09/927,732; 09/927,136; 10/044,522; 10/077,654;10/076,171; 10/075,896; 10/075,727; and 10/075,507, which are herebyincorporated by reference. Briefly, these applications disclose testmethods utilizing narrowband resonant circuits that accurately measureand characterize the loss due to use of a ferroelectric material, andestablish that ferroelectric components are not as lossy as previouslythought. Previous testing methods and devices did not account for allloss mechanisms and it was therefore not possible to accuratelydetermine the loss due to use of ferroelectric material. In oneimplementation, the narrowband resonant circuit used for testing is amicrostrip resonator having a gap to define the capacitor, and aferroelectric film deposited in the gap.

[0032] By employing proper testing methods and loss accountingmechanisms, tunable ferroelectric components can be optimized anddesigned for use in a wide variety of low loss applications andfrequency agile circuits. The choice of topology is critical forattaining the best possible Q (lowest losses). Depending on theparticular topology and materials that are employed, and the applicablefrequency range, Qs of greater than 80, greater than 180 and even higherare attainable. Design procedures and implementation details are setforth for gap capacitors, overlay capacitors and interdigitalcapacitors. The lowest losses are achieved by direct fabrication of theferroelectric capacitor to the resonator or other RF circuitry. Thiseliminates added losses caused by attachment of the ferroelectriccapacitor to a circuit.

[0033]FIG. 4 depicts a ferroelectric tuned oscillator resonant circuit200 according to a first embodiment of the present invention. VCO 200 issimilar to VCO 100 depicted in FIG. 1, but utilizes a ferroelectriccapacitor FE1 rather than a varicap diode D1. Ferroelectric capacitorFE1 is constructed, tested and integrated into circuit 200 as describedin the applications noted above and incorporated by reference. L1 is aninductor or other resonant transmission line device. The output ofcircuit 200 leads to an oscillator sustaining amplifier (not shown).

[0034] A variable ferroelectric capacitor has several advantages over avaricap diode. First, it has a much lower series resistance, typicallyby a factor of ten. This will directly result in a higher loaded Q(Q=X_(c)/R_(s), see discussion above) and better phase noiseperformance. Secondly, as plotted in FIG. 5, the capacitance/appliedvoltage transfer function of a ferroelectric capacitor is essentiallylinear, thereby permitting the design of more optimum PLL loop filters.Finally, ferroelectric capacitors do not suffer from the forward biasconduction problems of varicap diodes. This final aspect permits thedesign of novel band-switching oscillators, as will be described below.

[0035] In addition to setting the desired operating frequency, it isalso possible to use a combination of ferroelectric variable capacitorsto adjust the oscillation sustaining feedback path. This secondembodiment of the invention is illustrated in FIG. 6. By simultaneouslyadjusting the level of feedback in conjunction with changing thefrequency, the performance of the oscillator is maintained over a widerbandwidth. A conventional oscillator, such as the oscillator depicted inFIG. 1, has an optimum operating frequency where its output is at itsbest performance, in terms of spectral purity and RF power. This issometimes described as the oscillator's “sweet spot”. Maintaining thisoptimum feedback is important, especially for oscillator designs thatare required to tune across a wide bandwidth (greater than fifteenpercent of the oscillator center frequency). As the oscillator is tunedaway from its sweet spot and moves towards its limits of tuning,however, the RF output power frequently drops and the phase noise (orspectral purity) is degraded.

[0036] As illustrated in FIG. 6, it is possible to use a secondferroelectric component to maintain optimum performance across a widerbandwidth by varying the feedback parameters with the desired operatingfrequency. In order for oscillator 300 to produce a comparativelyspectrally pure signal, the open loop peak gain of the oscillator mustcorrespond to the zero phase shift point around the entire loop. Afailure to maintain this relationship across the tuning range will leadto degraded oscillator performance. In an extreme case, the oscillatormay stop oscillating.

[0037] In circuit 300 of FIG. 6, FE7 and FE9 are ferroelectric variablecapacitors. Capacitor C8 is a DC blocking capacitor, but could alsooptionally be a ferroelectric capacitor. Capacitors C10 and C11 are DCblocking capacitors. In operation, control voltage V4 is used to varythe capacitance of ferroelectric capacitor FE9. The capacitance ofcapacitor FE9, in conjunction with the other components (primarilycapacitor C8 and inductor L3) sets the oscillator frequency that isoutput by amplifier A1. Varying control voltage V4 thereforeproportionally varies the oscillator frequency. Control voltage V4, inaddition, controls the DC potential across ferroelectric capacitor FE7.Hence, the capacitance of FE7 also varies with the applied controlvoltage. By careful design and selection of capacitors C7, C10 and C11,the amplitude and phase of the feedback signal is accurately controlledat the desired frequency, thereby maintaining optimum performance.

[0038] A third embodiment of the invention, illustrated in FIG. 7,utilizes ferroelectric capacitors to provide a novel band-switchingoscillator 400. Circuit 400 can be configured in a number of ways toprovide effective band-switching or to affect the available tuningrange. Capacitors FE3 and FE4 are ferroelectric components, andcapacitors C5 and C6 are DC blocking capacitors. The output line fromcapacitor C6 leads to the oscillator sustaining amplifier (not shown).

[0039] Circuit 400 has two control voltage inputs: V2 and V3. Thefrequency of oscillation produced by circuit 400 can be affected byvarying control voltages V2 and V3 either together or independently.Multiple scenarios are possible. In a first scenario, a single (thesame) control voltage is applied to both V2 and V3. This makes thecontrol voltage across capacitor FE4 effectively zero, thereby settingits capacitance to its maximum value. Thus, the voltage across FE3 andhence its capacitance effectively dominate the frequency setting. In asecond scenario, control input V3 is grounded and a control voltage isapplied to V2. In this scenario, there is no potential difference acrossFE3 and its capacitance therefore is set to its maximum value. Thevoltage across FE4 and hence its capacitance effectively dominate thefrequency setting. Hence, by choosing different capacitance ranges forFE4 and FE3, effective band-switching can be provided by setting thecontrol voltages as described in the first and second scenarios.

[0040] In a third scenario, a control voltage is applied to V3 andcontrol voltage V2 is grounded. In this scenario, both FE3 and FE4experience the same DC control voltage and thus shift in capacitance.From an AC aspect, the capacitors are effectively in series, therebysignificantly increasing the operating frequency and permitting coverageof other communication bands. Similar results may be obtained byapplying different control voltages to V2 and V3.

[0041] Other embodiments and implementations of the invention will be orwill become apparent to one with skill in the art. All such additionalembodiments and implementations are intended to be included within thisdescription, to be within the scope of the invention and to be protectedby the accompanying claims.

1. A voltage controlled oscillator having a resonant circuit forgenerating a tuning frequency, the resonant circuit comprising: aninductive element; a ferroelectric capacitor having a variablecapacitance; and a control line coupled to the ferroelectric capacitorfor applying a control voltage to the capacitor, the control voltagevarying the capacitance which, in turn, varies the tuning frequency ofthe resonant circuit.
 2. An oscillator as claimed in claim 1, whereinthe control voltage has a linear relation to the capacitance across thetuning range of the capacitor.
 3. An oscillator as claimed in claim 2,wherein the resonant circuit has a relatively high loaded Q.
 4. Anoscillator as claimed in claim 3, wherein the resonant circuit has aloaded Q of at least
 180. 5. A voltage controlled oscillator comprisinga resonant circuit, the resonant circuit comprising a firstferroelectric capacitor configured to generate a variable resonantfrequency.
 6. An oscillator as claimed in claim 5, wherein the resonantcircuit further comprises a second ferroelectric capacitor configured tofacilitate frequency band-switching.
 7. An oscillator as claimed inclaim 6, wherein the first capacitor is coupled between ground and afirst control voltage, and the second capacitor is coupled between thefirst control voltage and a second control voltage.
 8. An oscillator asclaimed in claim 5, and further comprising a second ferroelectriccapacitor positioned in a feedback path of the oscillator to control theamplitude and/or phase of a feedback signal.
 9. An oscillator as claimedin claim 8, wherein a first control voltage is applied to both the firstand second ferroelectric capacitors.
 10. A voltage controlled oscillatorcomprising: a resonant circuit having a first variable ferroelectriccapacitor to generate a signal having a variable resonant frequency; anamplifier coupled to the resonant circuit to amplify the signal; and afeedback loop coupled between the amplifier and the resonant circuit andcomprising a second ferroelectric capacitor to control the amplitude andphase of a feedback signal.
 11. A voltage controlled oscillator asclaimed in claim 10, wherein a first control voltage is coupled to thefirst and second ferroelectric capacitors.
 12. A band-switchableoscillator resonant circuit comprising first and second ferroelectriccapacitors and first and second control voltage lines.
 13. A resonantcircuit as claimed in claim 12, wherein the first control voltage lineis coupled to the first and second ferroelectric capacitors, and whereinthe second control voltage line is coupled to the second ferroelectriccapacitor.
 14. A method for band switching in a voltage controlledoscillator comprising: providing first and second ferroelectriccapacitors; applying first and second control voltages to the first andsecond capacitors so that either the first capacitor or the secondcapacitor dominates the output frequency of the oscillator.
 15. A methodas claimed in claim 14, wherein the first control voltage is coupled toboth the first and second capacitors, and wherein the second controlvoltage is coupled to only the second capacitor.
 16. A method as claimedin claim 15, wherein the first control voltage and the second controlvoltage are approximately the same, causing the voltage across thesecond capacitor to be effectively zero and causing the first capacitorto dominate the output frequency.
 17. A method as claimed in claim 15,wherein the first control voltage is grounded and the second controlvoltage is not grounded, causing the voltage across the first capacitorto be effectively zero and causing the second capacitor to dominate theoutput frequency.